Composite materials enable mechanical characteristics to be obtained that are perfectly controlled and comparable to those obtained using metal, while being considerably lighter. Furthermore, the manufacturing techniques of these materials are continually being improved, both in terms of the shapes and measurements of the parts and the length of manufacture time and cost. It is for these various reasons that composite structures are being used more and more frequently in aircraft.
However, compared to metal structures, composite structures have the drawback of being more fragile when they are subjected to impact. When a composite structure is violently hit by an object variable degrees of breaking or cracking occur in the zone of impact. The increase in the number of composite structures used in aircraft has therefore led to significant improvements in the techniques used to repair these structures.
Two criteria should be adopted when the quality of a repair on an aircraft composite structure is evaluated: the appearance of the repaired zone and the mechanical characteristics of the structure in this zone. Furthermore, it is essential for the repair work to be carried out on-site, i.e. in the place where the aircraft is located, in order for costs to be reduced and to limit aircraft downtime to a minimum. In practice this rules out the use of autoclaves.
Appearance is an important criterion given that the damaged zone is usually located on the outside of the aircraft, i.e. in a zone that can be seen. It is therefore preferable for the zone to resemble the composite structure before it was damaged, such that it is practically invisible.
Moreover, the larger the damaged zone, the more crucial it becomes for the mechanical characteristics of the repaired zone to be as close as possible to those initially present in the composite structure before it was damaged.
At present a number of devices exist that enable composite structure repairs to be carried out on-site when the damaged zone is relatively small. These devices include tooling for step machining or scarf machining a recess in the damaged zone of the composite structure and tooling used to polymerize a composite part placed in the recess under pressure.
American patents 5 207 541 and 5 271 145 concern machining tooling. Tooling of this kind is generally used to machine circular- or oval-shaped recesses. However, other shapes are also possible, as disclosed by American patents 4 916 880 and 4 978 404.
The composite part set into a recess as described is of a shape that matches that of the recess. The composite part generally comprises several layers or folds of long fibre fabric and non-polymerized resin. If appropriate, the fabric may previously be impregnated with resin or be initially dry. If the fabric is dry the resin can either be applied as a liquid on each layer of fabric or as solid film interposed between the various layers of dry fabric.
Generally, the tooling used to polymerize and compact the composite part mainly comprises a heating cap that raises the resin to the temperature at which it polymerizes and a bladder fastened around the composite part such that it forms a leaktight seal on the surface of the composite structure. The bladder is connected to an external source of negative pressure such that pressure may be applied to the composite part.
As shown in American patent 4 554 036, the pressure applied to the composite part in this standard tooling remains very reduced, for example approximately 1.5 bar.
This kind of tooling can therefore be used for "cosmetic" repairs, in other words for reconstituting the original appearance of the composite structure. However, this tooling affects the mechanical characteristics of the repaired zone and can reduce the characteristics by up to 30% compared to the structure in its original condition. Repair work using this technique is, therefore, limited to only small damaged zones.
When larger zones are damaged the damaged composite structure is usually completely replaced and then sent to be repaired in a specialist workshop equipped with autoclaves that perform high pressure polymerization under appropriate conditions.
In another known technique a composite part is molded in an autoclave into a shape that matches that of the recess previously machined in the damaged zone. The composite part is then polymerized and compacted in an autoclave before being bonded into place in the recess. This technique is discussed in American patent 5 023 87.
The major drawback of this technique is that the ability to withstand fatigue in the bonded zone is relatively limited, thus constituting a particular drawback for the aeronautics industry. It is for this reason that the fastening of the additional composite part is generally completed by other fastening means, such as bolts or rivets, that require access via the rear surface of the structure. The repair work can be seen from the outside and does not, therefore, meet the above-mentioned requirements concerning appearance.
Furthermore, the polymerized composite part is manufactured in an autoclave, i.e. generally on a different site from that where the aircraft is located. This means that the aircraft will be immobilized for a long and costly period.